One important area of application for the invention is for wavefront measurements of high-resolution projection lenses in microlithography for semiconductor wafer patterning so as to be able to determine aberrations of the projection lens with high precision. As is known to the person skilled in the art, it is possible for this purpose to use, for example, a technique based on lateral shearing interferometry or other interferometry techniques such as point diffraction interferometry (PDI) or line diffraction interferometry (LDI). Also possible is the use of a Shack-Hartmann sensor or a sensor based on moiré techniques.
In one embodiment of shearing interferometry, a so-called coherence mask is placed in the object plane of the optical system to be examined. An object pattern is arranged thereon. A reference pattern designed as a diffraction grating is located in the image plane of the imaging system. Due to the superposition of the waves produced by diffraction at the diffraction grating, a superposition pattern in the form of an interferogram is produced, which is captured using a suitable detector. Possible embodiments of a coherence mask and of a diffraction grating of a shearing interferometer are specified, for example, in DE 10 2005 041 373 A1.
However, reproducibility and absolute accuracy of the wavefront measurements carried out using conventional shearing interferometry methods are often insufficient, in particular in the case of optical systems designed for EUV radiation.
Another area of application of the invention is for determining pupil-resolved transmission behavior of high-resolution projection lenses in microlithography, also referred to as “apodization.” Here, a spatially resolving detector, which is arranged below the image plane of the projection lens, is used to record an intensity distribution. The accuracy of the measurements carried out using conventional methods is here often also insufficient.